1. Field of the Invention
This invention relates to optical semiconductor device packages and optical semiconductor devices housing optical semiconductor chips, and in particular to optical semiconductor device packages and optical semiconductor devices in which characteristic impedances of leads are reduced.
2. Description of the Background Art
Recent years have seen an increase in transmission rate in optical communications, up to 10 Gbps or faster in some cases. With this, there have been demands for optical semiconductor chips as well as optical semiconductor device packages housing them that are relatively inexpensive and have excellent high frequency characteristics.
As a structure that satisfies such demands, Japanese Patent Application Laid-Open No. 2004-311923 (FIG. 4) discloses a stem type optical semiconductor device package in coaxial form having a simple structure.
The stem type refers to a structure that includes a disc-shaped substrate called a stem, a plurality of leads penetrating the stem in the direction of the thickness to protrude from a main surface of the stem, and a base at ground potential called a mount that extends vertically from the main surface of the stem, with an optical semiconductor chip being mounted on the mount.
In the stem type optical semiconductor device package, an optical semiconductor chip is mounted not directly on the mount, but on a substrate called a sub-mount that is provided in direct contact with the mount. The optical semiconductor chip and leads are electrically connected by wire.
The stem-penetrating parts of the leads are covered with glass, but the protruding parts from the stem main surface are not covered for the purpose of wire connection. This causes an increase in characteristic impedance of the uncovered parts of the leads, and the resultant characteristic impedance mismatching causes an increase in reflection, thus deteriorating the reflection characteristics. This leads to a reduction in transmission rate.
Therefore, the above Japanese Patent Application Laid-Open No. 2004-311923 discloses, in FIG. 4 for example, a structure in which the characteristic impedances of leads are reduced by providing a dielectric plate on a mount surface facing the leads.
As has been discussed, in the conventional stem type optical semiconductor device package, the characteristic impedances of leads have been reduced by interposing a dielectric plate between the leads and mount. The characteristic impedances in this case are defined by the distance between the leads and mount, which depends on manufacturing accuracy, and the permittivity of the dielectric plate, which depends on the material of the dielectric plate. Therefore, only a low degree of flexibility and a limited range are given in controlling the characteristic impedances.